Windows instances failed to setup network.

I am having some weird issue with creating Windows instances on OpenStack Ocata.

We are creating our instances in the external network, and used to work fine in Liberty.

The issue we are seeing is that about half of the instances fail to connect to Metadata server because the Cloudbase MTUPlugin doesn't set the MTU value to 1450(By comparing the cloudbase log between failed instances and functioning instances).

The Cloudbase version is 0.9.9, and we use openstack self-service network setup based on the Ocata doc.

Another weird thing we see is that creating instance is working under the same subnet where compute nodes and controller node stay, but about half instances malfunction if it's in a different subnet.

Consider we put all nodes in 10.10.136.0/24, VLAN is 10.10.136/22, and instances in 10.10.137.0/24, 10.10.138/24 and 10.10.139.0/24 are having this issue.